System for cleaning contaminated soil

ABSTRACT

Water contaminated with halogenated organics, including chloroform, trichloroethane, solvents, pesticides, etc. is treated by passing the water through a permeable mixture of activated carbon and iron fillings. When the mixture is brought to a negative Eh voltae, the metal causes the contaminants to undergo chemical breakdown. The activated carbon acts to retard the contaminant, giving it a long residence time close to the iron. The negative Eh conditions demand oxygen exclusion, such that a favoured application is to place the mixture in a trench below the water table in an aquifer, in the path of a plume of contaminant. The mixture may also be contained in a tank above, or in, the ground. An inert filler material, such as sand, may be included in the mixture.

This invention relates to the cleaning of contaminated water.

The invention is mainly concerned with the cleaning of groundwater inits native aquifer, or of effluent water discharged from a manufactory.

The invention is not limited in scope only to specific contaminants, butthe contaminants with which the invention is mainly concerned are thosewhich are carcinogenic or otherwise hazardous in trace quantities, andwhich are difficult to break down chemically. A typical example of thetype of contaminant is the halogenated hydrocarbon (organic) type, whichincludes eg chloroform; solvents such as carbon tetrachloride; andpesticides such as DDT.

THE PRIOR ART

One of the well-known conventional systems for removing contaminantsfrom water is to pass the contaminated water through a body of activatedcarbon. Activated carbon is highly adsorptive material, whereby thedissolved contaminants are removed from the water and retained on theactivated carbon.

Over a period of time, the contaminant builds up on the activatedcarbon. One way of dealing with the activated carbon that has becomesaturated with the contaminant is to periodically remove the activatedcarbon, and dispose of it as a hazardous waste.

Alternatively, periodically the activated carbon may be flushed orotherwise treated (eg by heating) to remove or drive off thecontaminated material that has accumulated. When the activated carbonhas been flushed it can be re-used. The contaminants however stillexist, and must be disposed of.

Thus, one of the disadvantages of the conventional decontaminationsystems based on the use of activated carbon is that the contaminantremains intact. In the conventional system, the activated carbonfunctions simply to strip the contaminant out of the water; thecontaminant material that is removed from the activated carbon uponflushing is still hazardous. In fact, it is even more hazardous becauseit is concentrated. The contaminant may finally be broken down in afurther treatment facility, or it may be disposed of as a hazardouswaste.

It is an aim of the invention to provide a decontamination system inwhich contaminants, such as those of the halogenated organic type, arebroken down into harmless, or at least less hazardous, chemicalsubstances.

It is also known in the art to pass an halogenated-organic contaminantthrough a permeable body comprising a pair of metals. It has been foundthat the halogenated-organic materials break down, when in prolongedcontact with the pair of metals, inferredly by a form of hydrolysisreaction, into chlorides etc; these substances generally are virtuallyharmless in trace quantires, and often will precipitate out of the wateras insoluble solids.

One of the disadvantages with the above "metals" system is thatsubstantial periods of time, and/or substantial quantities of the metalsare required. The system can be expensive, not only as regards providingthe metals, but also as regards arranging for a sufficient residencetime of the water within the body of metals, and as regards creating thebest conditions of pH level, temperature, oxidizing/reducing conditions,etc, throughout that residence time.

The great advantage of the "metals" system is that the hazardousmaterial disappears.

It is an aim of the invention to provide a decontamination system inwhich, as in the known "metals" system described above, the hazardousmaterials are broken down and converted into relatively harmlesschemical compounds; it is also an aim to achieve this breakdown of thecontaminants using substantially smaller quantities of metal.

GENERAL FEATURES OF THE INVENTION

In the invention, the contaminated water is passed through a permeablebody comprising a mixture of an adsorptive material, such as activatedcarbon, and a metal, such as iron.

It is recognized that the function which the adsorptive materialperforms when mixed with the metal is substantially different from itsconventional function as a mere adsorber of the contaminant. Rather, itmay be regarded that the adsorptive material acts as a retarder, toretard the passage of the contaminant, and to keep the contaminant inclose proximity to the metal for a very long period. The effect is toincrease the residence time in which the contaminant remains in contactwith the metal, just as if a very large body of the metal had beenprovided.

The water itself may pass through the body in a short time period, asdetermined by the permeability of the body, the differential pressures,etc.

When the body did not include an adsorbing material, as in theconventional "metals" system, the contaminants were retarded hardly atall, and travelled through the body in virtually the same time period asthe water. (It may be noted that many dissolved contaminants and othersubstances are naturally retarded to some degree, relative to the waterin which they are dissolved, when passing through an aquifer. But thisnatural retardation, which indeed is not always small, is of littlesignificance when compared with the retardation that is attributable tothe activated carbon or other adsorption material, in the invention.)

It was known in the conventional systems to mix the metals with sand orother inert filler material, whereby the bulk of the body through whichthe water had to pass could be increased; and this measure had someeffect in increasing the residence time. However, the increase inresidence time was more or less simply proportional to the increase inbulk: in the invention, wherein the metal is mixed with an adsorptivematerial such as activated carbon, the residence time may be increasedby an order of magnitude, or more, greater than would be accounted forby the simple increase in bulk volume.

In the invention, the adsorptive material is not in fact acting as amere adsorber, since the contaminant is broken down: nor is theadsorptive material acting simply as a bulk filler material. Theadsorptive material co-operates with the metal with which it is mixed,to retain the molecules of the contaminant substances in contact withthe metal for a long period, thereby aiding in the effectiveness of themetal in breaking down the contaminant. The adsorptive material does notact to retard the flow of water through itself, but rather theadsorptive material acts to retard the movement of the contaminantmolcules so that those molecules spend a considerable time in thepresence of the metal.

It is recognized that the performance of the mixture of adsorptivematerial plus metal, as regards the quantity of metal needed todecontaminate the water, and the rate at which the water can bedecontaminated, far exceeds that of the metal by itself.

In order for the metal to be effective in causing the chemical breakdownof the contaminant, the reduction/oxidation condition of the metal mustbe under strict control. As measured by an Eh probe, the Eh voltage mustdrop below -200 millivolts, and preferably below -600 mv, for thebreakdown to occur. Therefore, the mixture, and the water passingthrough it, must be kept under strictly anaerobic conditions. Whateveravailable oxygen is present in the water, or in the materials of themixture, will lead to time-inefficiencies, because such available oxygenwill have to be consumed in order for the Eh voltage to become negative,and before the degradation of the contaminant can commence.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

By way of further explanation of the invention, an exemplary embodimentof the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a cross-section through an aquifer, the water in which isbeing treated in a manner which embodies the invention;

FIG. 2 is a diagrammatic view of another water treatment facility whichembodies the invention;

FIGS. 3A and 3B and FIG. 4 are graphs which illustrate the manner ofdegradation of a typical contaminant as the water flows through amixture of activated carbon and metal.

The apparatus and procedures shown in the accompanying drawings anddescribed below are examples which embody the invention. It should benoted that the scope of the invention is defined by the accompanyingclaims, and not necessarily by specific features of exemplaryembodiments.

The aquifer 2 shown in FIG. 1 is saturated with water up to the level 3of the water table. The groundwater is moving through the aquifer with avelocity VW. A plume 4 of a contaminant is moving along with the water.The velocity of the contaminant will tend naturally to be somewhatretarded relative to the groundwater, in its passage through theaquifer, whereby the contaminant in the plume is moving through theaquifer with a slower velocity VP.

The contaminant is of the halogenated hydrocarbon (organic) type, whichterm includes, for example, carbon tetrachloride. The contaminant mayhave arisen because of a spill of known origin that must be cleaned upby those responsible, or the contaminant may be from an unknown sourcebut has been detected as heading for a well, for example.

A trench 5 is excavated down into the material of the aquifer 2, in thepath of the plume 4. Into the trench 5 is placed a permeable bodycomprising a mixture 6 of activated carbon and iron filings, andsand/gravel. The mixture is of such permeability that the flow ofgroundwater is not impeded by the permeable body: preferably, thepermeability of the mixture 6 should not be less than the permeablity ofthe aquifer material.

The trench 5, and the mixture 6 therein, should extend laterally as faras is necessary to ensure that all the contaminated water flows throughthe mixture. As to height, the trench and the mixture should be soplaced as to intercept the whole height of the plume. The mixture neednot occupy the portion of the trench 5 that lies above the water table3; this portion may be filled in with sand or other filler material.

FIG. 2 shows another system for treating contaminated water.

In the FIG. 2 system, the contaminated water is piped via inlet andoutlet pipes 7,8 through a vessel 9. The vessel 9 contains a mixture 10of activated carbon and iron filings. The vessel 9 may comprise a tank,which is placed on, or buried in, the ground: if natural circulation isnot available, a pump (not shown) may be provided to feed the waterthrough the system. In FIG. 2, the contaminated water may be, forexample, effluent from a factory, or groundwater that has been taken outof the ground, or water that has been drawn off from a well, etc.

It is recognised that the metal in the mixture, whether iron or anothermetal, is effective to initiate and promote the chemical breakdown ofthe halogenated hydrocarbon contaminants which lie in close proximity tothe metal, and which lie there for a sufficient period of time, andunder the correct conditions of pH, temperature, salinity, and so on.One of the major conditions is the exclusion of all oxygen andoxygen-supplying agents from the mixture and from the water, as will nowbe discussed.

In the FIG. 1 system, the mixture, and the water, all lie below thewater table 3, and it can be expected that the natural conditions willtherefore be substantially anaerobic, without any precautions needing tobe taken. The presence of the metal can therefore be expected to drivedown the Eh voltage of the water entering the mixture in the trench in areasonably short period of time, so that breakdown of the contaminantwill quickly commence.

Not much can be done, in any event, with flowing groundwater, to changesuch conditions as pH, temperature, presence of other dissolved orsuspended substances, etc, so that the system of FIG. 1 is onlyapplicable in those cases where conditions happen to be right naturally.The correct conditions often do prevail, however.

The system of FIG. 2 is more versatile, though less economical. Thevessel 9 should be airtight, all oxygen, or oxydising agents, beingexcluded. Conditions in the vessel may be monitored, and adjustmentsmade to temperature, pH, and so on, as may be required. Conventionalinstruments, and apparatus and procedures, exist for detecting the needfor, and effecting, such adjustments, and are not described here.

In the conventional systems where water is passed through a body ofactivated carbon, the water passes through the body, typically, in a fewminutes. The larger the body of activated carbon, and the longer theresidence time of the water therein, the more molecules of thecontaminant would be expected to be taken out of the water--though ofcourse, in the conventional systems, the contaminant would remain intacton the activated carbon. The contaminant would gradually build up on theactivated carbon until the activated carbon became saturated, and nomore contaminant could be adsorbed from the water.

In the conventional system, the rate of flow of water that could betreated by a given body of activated carbon was determined by the rateat which the contaminant could be adsorbed into the activated carbon: inthe invention, in many real practical applications, the rate at whichthe contaminant is chemically broken down can even be faster than therate at which the contaminant, in the conventional systems, was merelyadsorbed.

It may be regarded that whatever molecules of the dissolved contaminantare adsorbed from the water will not be released and will not escapeinto the water outflow, and will eventually be broken down due toproximity to the iron: therefore, if the residence time of the water inthe activated carbon is sufficient to extract substantially all themolecules of the contaminant, then substantially all of the contaminantwill be broken down.

The metal that is used in the mixture preferably is iron, since iron iswidely available in granular form inexpensively, as waste from manyprocesses. The grain size of the granules of the metal should be assmall as possible, in order that the granules may have a maximumreactive area.

On the other hand, the metal should not be in the form of so fine a dustas would make it difficult to handle.

The metal need not be elemental, so that steel or cast iron granules maybe used, rather than pure iron. The metal selected for use in theinvention should not be of a very low electrochemical activity: silveror gold, for example, would not be effective. Metals such as zinc, iron,aluminum, are candidates for selection on the basis of their electro-chemical activity, and considerations of practical availability willusually favour iron, as mentioned.

The presence of oxide on the metal is generally detrimental, andpro-treatment, for example an acid wash, is usually to be recommended toremove at least some of the oxide and expose the metal.

It has been found that sometimes the speed of the reactive effectattributable to a metal may be affected by the presence of otherelectrochemically active metals: for example, if galvanized iron is usedas the source of the granules, the breakdown rate can be expected to beslightly slowed by the presence of the zinc, as compared with iron byitself. Also, it has been found that granules of stainless steel are notso effective as granules of ordinary carbon steels.

In some cases, it has been proposed that certain pairs of metals, mixedor alloyed together, will out-perform a single metal in breaking downsuch contaminants as the halogenated hydrocarbons. It should beunderstood that the invention may be used to advantage when the metal inthe mixture is in fact such a pair of metals.

FIG. 3A is a graph which plots the gradually falling concentration of acontaminant under certain conditions. In the model system represented byFIG. 3A, water containing 1400 parts per billion of trichloroethylene(TCE) was passed through a permeable body. The permeable body was in theform of a column, of a length of 0.6 meters.

The body was a mixture comprising 10% (by mass) iron filings, 0.5%activated carbon, and the rest of the mixture was silica sand.

The conditions were such that the water travelled through the mixture ata velocity of 318 cm/day, or 13 cm per hour.

The water entering the mixture had a concentration of TCE of, as shown,1400 parts per billion. Plot 25 is the plot of the concentration of TCEin the water after a steady state had been reached. The points whichdefine the plot were measures of the TCE concentration at the particularpoints in the column. As may be seen, the water at the 40 cm markcontained practically no detectable TCE. The legal limit of TCE indrinking water, in some jurisdictions, is 30 ppb, and water beyond aboutthe 15 cm mark was within this limit.

For comparison, plot 26 shows the effect of omitting the metal, ie ofsimply adsorbing the TCE out of the water, and onto the activatedcarbon. At first, the contaminant was removed from the water at a ratewhich, though slower, was comparable with the rates shown in plot 25.

As the activated carbon became saturated with contaminant, the rate atwhich TCE was taken out of the water started to decrease, and when theactivated carbon was fully saturated, no further contaminant was removedat all. Thus the final steady state of the plot 26 would be the line26F.

It should be noted that, unlike plot 25, plot 26 merely represents theloss of the contaminant from the water: the contaminant itself remainedintact on the activated carbon, so that the activated carbon becamegradually more saturated. Thus, when the contaminant was merely adsorbedonto the activated carbon, the "front" of contaminant would progressgradually along the column, until finally no further adsorption tookplace. When the contaminant is chemically broken down, on the otherhand, a steady state condition becomes established wherein theeffectiveness of the mixture does not diminish.

Tests with other contaminants show a similar pattern. In the modelsystem represented by FIG. 3B, water containing 1400 parts per billionof carbon tetrachloride (CTC) was passed through the permeable body.Again, the body was a mixture comprising 10% (by mass) iron filings,0.5% activated carbon, and the rest of the mixture was silica sand, andthe conditions were such that the water travelled through the mixture ata velocity of 318 cm/day, or 13 cm per hour.

The water entering the mixture had a concentration of CTC of, as shown,1400 ppb. Plot 27 (FIG. 3B) shows the concentration of CTC, which, asshown, dropped very quickly to zero: however, the contaminant at thisstage had not disappeared as a hazardous substance but had merely beenconverted into chloroform. Plot 28 shows the chloroform concentration,showing that after the contaminated water had travelled about 2 cm intothe mixture, the concentration of chloroform rose to a maximum of 700ppb, as the CTC dropped to zero. After that, the chloroform graduallybroke down, and by the time the water had reached the 30 cm mark nochloroform could be detected in the water. The plots 27,28 represent asteady state condition, in which the rate at which the mixture brokedown the contaminant remained undiminished for long periods.

The plots 25 and 27,28 show the effect by which the contaminants undergochemical breakdown, down to virtual disappearance. It should be notedthat these plots represent steady state conditions: the whole flow ofthe water that entered the permeable body continued to be contaminatedat the 1400 ppb level, ie the contaminant was not just supplied as asingle pulse.

FIG. 4 is a graph that shows the effects of including mere activatedcarbon in the mixture. Plot 35 shows the rate of chemical breakdown ofTCE when no activated carbon was present in the mixture, ie when onlymetal, in this case iron filings, was present. Now, the contaminant issubstantially not retarded at all, but passes through the metal at thesame velocity as the water. When activated carbon is included, and theproportion increased, the retardation effect occurs: the plot thereforemoves to the left in FIG. 4. Thus, plot 36 represents the rate at whichTCE breaks down when the contaminant moves at half the velocity of thewater. The plots 37-39 represent progressively mere activated carbon inthe mixture. The proportions required to achieve the desired degree ofretardation vary according to conditions, but the said mixture of 10%iron: 0.5% activated carbon has, as described, been effective. The levelof retardation shown in plot 39 may, depending on conditions, representa mixture of, for example, equal quantities of activated carbon and ironfilings.

An upper limit to the proportion of activated carbon might, in practice,apply in some cases. If too much activated carbon were provided, themolecules of contaminant taken out of the water might be so stronglyadsorbed into the activated carbon that the molecules became protected,by the fact of their adsorption into the activated carbon, from thebreakdown action of the metal. The quantity of, and the physical spacingof the granules of iron, in relation to the granules of activatedcarbon, is important: there should be enough granules of iron that thegranules of iron are physically close enough to each other to exert theEh lowering effect, and the chemical break-down influence, throughoutthe activated carbon.

It should be noted that the mixture includes also the inert fillermaterial, silica sand. Apart from simply providing bulk, the silica sandserves to prevent consolidation, which might be expected to occur,especially in the metal, over long periods and which might lead to localnon-homogeneities in the permeability of the mixture.

The plots of the rate of contaminant breakdown would be expected to moveto the right in FIG. 4 in the case where the nature of the contaminant,or the prevailing conditions of temperature, pH, etc, give rise to aslower characteristic breakdown rate. The proportions of activatedcarbon to iron filings, and the quantity of bulk filler material needed,therefore do need to be tailored to the particular conditions, as willbe determined by local tests.

As mentioned, in the invention the contaminant does not build up on theactivated carbon by progressive continuing adsorption. Since thecontaminant does not remain adsorbed into the activated carbon, theactivated carbon can be expected to last indefinitely. The metal howeveris gradually used up in the process of breaking down the contaminant,and after a time the metal would need to be replaced.

It would be possible to add new metal into the vessel shown in FIG. 2,though it would hardly be practical to add new metal into a trench asshown in FIG. 1. When the trench version is specified, a margin of extrametal should be included in the trench. On the other hand, excavating atrench does not entail a huge expense, and to provide a second trenchlater, if the first proved inadequate, would often not be a problem.

One disadvantage of the "metals" system, which is shared by theinvention, is that, although the halogenated organic materials aredestroyed, the metals themselves sometimes can cause the water to becometainted. This is especially important if the water is to enter adrinking-water supply system soon after being treated. If the treatedwater is to spend a long period passing slowly through an aquifer,though, the problem of tainting by the metals can be expected to beinsignificant.

I claim:
 1. Procedure for treating contaminated water, by passing thewater containing contaminant in solution through a permeable body oftreatment material comprising particles of an adsorptive materialphysically mixed with particles of a metal, wherein:the nature of thecontaminant and the nature of the metal are such that the contaminantbreaks down by chemical reaction into chemically distinct and differentsubstances when brought into, and during the course of, prolongedcontact with the particles of metal; the nature of the adsorptivematerial is such that the contaminant is adsorbed out of solution ontothe particles of adsorptive material upon the contaminated water beingpassed over and through the permeable mixture; the adsorptive capacityof the adsorptive material is such that the velocity of the contaminantpassing through the permeable mixture is substantially more retardedthan the velocity of the water passing through the permeable body;whereby the contaminant, being retarded on and by the parities ofadsorbent material, is held physically adjacent to the particles ofmetal for a substantially longer period of time than the passing water,and is so held long enough for chemical breakdown of the contaminant totake place; and the procedure includes the step of so disposing andarranging the mixture that all oxidising agents and materials, includingatmospheric oxygen, are excluded from contact with the mixture. 2.Procedure of claim 1, wherein the metal is in granular form. 3.Procedure of claim 1, wherein the adsorptive material is activatedcarbon.
 4. Procedure of claim 1, wherein:the water is groundwatercontained within its native aquifer, and the contaminant occupies aplume within the aquifer moving through the aquifer; the procedureincludes the steps of excavating a trench in the material of the aquiferacross the path of the moving plume; of placing the mixture within thetrench, the arrangement thereof being such that the plume ofcontaminated water passes through the mixture.
 5. Procedure of claim 1,wherein the mixture includes also an inert filler material.
 6. Procedureof claim 1, wherein the adsorptive material is activated carbon, and themetal is iron in the form of iron or steel filings, and wherein themixture comprises 10% by mass of the filings, 0.5% by mass of theactivated carbon, in a bulk of silica sand.
 7. Procedure of claim 1,wherein the adsorptive material is activated carbon, and the metal isiron in the form of iron or steel filings, and wherein the proportionsthereof in the mixture are between 1 part activated carbon to 20 partsmetal, and equal parts of activated carbon and metal, by mass, in a bulkof silica sand.
 8. Procedure of claim 1, wherein the said metal consistssolely of one single element.
 9. Procedure of claim 8, wherein the metalis iron.
 10. Procedure of claim 1, wherein the nature of the mixture issuch that the surfaces of the particles of metal are accessible fordirect exposure to the contaminated water, the surfaces beingsubstantially free of such coatings and inclusions as would, if present,inhibit the completion of the said chemical reaction.
 11. Procedure ofclaim 10, wherein the metal is bare, and the surface thereof is indirect wetting contact with the contaminated water.
 12. Procedure ofclaim 1, wherein the chemically distinct substances resulting form thebreak down reaction are substantially harmless.
 13. Procedure of claim1, wherein the contaminated water is groundwater in its native aquifer.14. Procedure of claim 1, wherein:the contaminated water is groundwaterwithin, and moving through, its native aquifer; the procedure includesthe step of placing the said permeable body of treatment material in theground, in the path of the moving contaminated groundwater, and ofcausing the contaminated groundwater to pass therethrough.
 15. Anapparatus for treating contaminated water, wherein:at least one of thecontaminants in the water is of the kind which breaks down by chemicalreaction into chemically distinct and different substances when broughtinto, and during the course of, prolonged contact with particles ofmetal; the apparatus includes a body of metal particles, and includes ameans for directing the flow of contaminated water through the saidbody; the apparatus includes a means for excluding oxidising agents andmaterials, including atmospheric oxygen, from the body of metalparticles, and the means is effective to exclude the said agents andmaterials; the apparatus includes a body of an adsorbent material, ofthe type which is capable of adsorbing the said at least onecontaminant; the body of metal particles and the body of adsorbentmaterial are mixed together to form a mixture, the mixture being suchthat the mixture is permeable to the flow of water therethrough; themixture is positioned within a flowing stream of thecontaminant-containing water; the adsorptive capacity of the body ofadsorptive material, and its disposition in the mixture, are such thatthe flow rate of the contaminant passing through the permeable mixtureis substantially more retarded than the flow rate of the water passingthrough the permeable mixture; whereby the contaminant, being retardedon and by the adsorbent material, is held physically adjacent to theparticles of metal for a substantially longer period of time than thepassing water, and is so held long enough for chemical breakdown of thecontaminant to take place.
 16. Apparatus of claim 15, wherein theapparatus includes also a body of an inert filler material, and theinert filler material is mixed into the permeable mixture.
 17. Apparatusof claim 15, wherein the contaminated water is groundwater flowingthrough an aquifer;the means for directing the water to flow through themixture comprises a trench excavated in the path of the contaminatedwater, and comprises the presence of the mixture in the trench; themeans for excluding oxidising agents and material comprises apositioning of the mixture below the water table in the aquifer, wherebythe mixture is saturated with the groundwater.